The invention relates to the technical field of tribology in lubricated regime.
More particularly, the invention relates to coatings and surface treatments for reducing wear and for minimizing the transmission of tangential forces.
Numerous technical solutions have been proposed for improving the tribological performance of mechanical parts. A distinction is made essentially between conventional case hardening treatments and deposits of thin and hard layers obtained by various methods such as PVD (Physical Vapour Deposition) or PACVD (Plasma Assisted Chemical Vapour Deposition).
Among the deposits of thin and hard layers, mention can be made of deposits of transition metal nitrides (TiN, CrN, TiAIN, etc.), coatings of amorphous carbon (DLC), etc. For technical, as well as mechanical reasons, it appears that these deposits of surface coatings do not generally exceed 5 μm. Above this thickness, risks of embrittlement of the layer and flaking may occur. It is also important to obtain perfect adhesion and strength of the thin layer over time. Thus, a person skilled in the art claims very slightly irregular surface textures with a roughness (Ra) of about 0.04 μm.
It therefore appears from the prior art that the tribological performance of the surface treatments and vacuum deposits are only guaranteed with surfaces having a slight roughness.
For example, mention can be made of the teaching of U.S. Pat. No. 6,886,521, which sets a maximum value of the surface roughness parameter (Rz) as a function of the hardness of the DLC deposit and the thickness thereof. A person skilled in the art generally claims the lowest possible roughnesses, that is, for example, Ra≦0.04 μm.
Thus, it appears from the analysis of the prior art that high surface roughnesses are unsuitable for receiving hard deposits, because this produces surface irregularities leading to local overpressures liable to cause excessive plastic deformation detrimental to the mechanical strength of the coating. Furthermore, in lubricated regime, when the amplitude of the roughnesses is too high, the oil film is locally broken and the surfaces which are in relative motion are no longer perfectly separated, thereby giving rise to an increase in the friction coefficient and premature wear of the said surfaces.
Scientific publications in the field of tribology also discuss the influence of roughness on lubrication. Mention can be made for example of: “Occurrence of microelastohydrodynamic lubrication in simple sliding motion with transverse roughness”—J. Wang, M. Kaneta, F. Guo, P. Yang—Journal of Engineering Tribology—Vol. 220 No. J13 (May 2006) pp. 273-285.
It appears from these publications that the amplitude of the surface roughnesses must be considered, with the conclusion that the lowest possible roughnesses must be sought in order to decrease the friction coefficient.
Also noteworthy, as it appears from the scientific publication: “Influence of harmonic surface roughness on the fatigue life of elastohydrodynamic lubricated contact”—A. D. Chapkov, F. Colin, A. A. Lubrecht—Journal of Engineering Tribology—Vol. 220 No. J13 (May 2006) pp. 287-294”, is that the numerical analysis of the rough surface contact treated as sinusoidal profiles, reveals overpressure effects associated with the irregularities of the contact surfaces, and indicates that these overpressures may be limited by increasing the profile period. However, this decrease in the contact pressure on high wavelength profiles, is explained by an elastic deformation of the substrate. In the case of hard deposits obtained by the PVD or PACVD method, these deposits, due to their high modulus of elasticity and their high hardness, cannot accommodate the deformation of the substrate, so that they are rapidly damaged.
It therefore appears clearly from the prior art, in the field of tribology, that the thin layer deposits, for example in the case of amorphous carbon, must be carried out on surfaces having very high finish levels with a roughness (Ra) not exceeding a few hundredths of a micron and generally less than 0.04 μm.